Energy transmission via electric power systems is critical to modern civilization. Since numerous electric devices draw alternating current (AC) power from utility sources (i.e., line power from the electric power grid), power failures (i.e., outages or interruptions) can be a serious concern. The use of uninterruptible power supplies (UPSs) having battery back-up systems to provide regulated, uninterrupted power for sensitive and/or critical loads, such as computer systems, and other data processing systems is well known.
Uninterruptible power supply (UPS) systems are commonly used in conjunction with a variety of electric devices to safeguard against power failures. UPS systems are available in various configurations to provide varying functionality, but they all typically provide power in the event of a power interruption.
A UPS system may be connected to line power so that a device connected to the UPS system (referred to as the load) may use line power during normal conditions. UPS systems typically include a battery to store energy, a rectifier to convert AC to direct current (DC) for passing through the battery, and an inverter to convert DC battery power back to AC when backup power is generated.
Ideally, the voltage waveform corresponding to the UPS output voltage (i.e., the electrical signal representing the voltage delivered to the load) is sinusoidal, since electric devices draw AC power that is sinusoidal. Nonsinusoidal waveforms (e.g., square waveforms) typically have greater occurrence of transient oscillations due to sharp voltage changes. However, certain nonsinusoidal waveforms, such as square waves, are easier to generate (and hence less expensive) than sinusoidal waveforms and are therefore commonly used in UPS systems.